Power supply controllers have been provided in which a semiconductor switching element for high power, for example a power MOSFET, is provided on a conduction channel between a power source and a load, and power supply to the load is controlled by turning on and off the switching element. Such power supply controllers having a self protection function are known. The self protection function controls the electric potential of a control terminal of the switching element and turns off the switching element if an overcurrent flows through the conduction channel, thereby protecting the switching element itself. Specifically, in a self protection function, such as the one described in Japanese Patent Laid-Open No. 2001-217696, a current detecting resistor is connected with a load terminal (for example the source or drain of a MOSFET) in series, a load current flowing through the switching element is detected on the basis of a voltage drop at a resistor and, if the load current exceeds a predetermined threshold, it is determined that an overcurrent has occurred. An electric current shutoff based on the self-protection function is designed in such a manner that the switching element automatically returns to the on state upon expiration of a predetermined after shutoff time period. This is because this function is provided in order to avoid overheat of the switching element itself, and as such, a heat sink provided for that purpose quickly decrease the temperature of the switching element after the abnormal current is shut off.
In a case where detection of an abnormal overcurrent is attempted by comparing a load current that flows to a switching element with the above described threshold, during the process from the time the switching element is turned on until it enters a stable state, a current flowing to that switching element varies along a predetermined load line. Therefore, when the aforementioned threshold is set at a constant level, there is a problem that, depending on the case, some time is required from occurrence of the abnormal overcurrent until detection thereof. For example, FIG. 10 is a view illustrating a flowing current Id and a voltage Vds between the drain and source of a power MOSFET as the above described switching element. If the load is in a normal state, by turning this power MOSFET on, the value of the current Id and the voltage Vds between the drain and source varies along the load line L0 from the point B0, and is stable at the time that it arrives at a stable point A0.
However, when an abnormal overcurrent such as a short-circuit of a load occurs, by turning on the power MOSFET, regarding the value of the current Id and the voltage Vds between the drain and the source, even if the value initially starts from the point B0, since a voltage drop for that load is extremely small the source voltage of the power MOSFET increases very little. More specifically, the current Id flowing through the power MOSFET rises rapidly in a state in which the voltage Vds between the drain and the source of the power MOSFET does not vary greatly. On the other hand, when the aforementioned threshold is set at a constant level, in other words, when a single threshold is used to detect abnormal overcurrents that may occur at each step from the time the power MOSFET is turned on until the current reaches a stable state, in order to detect an abnormal overcurrent after the stable point A0 is reached, it is necessary to set the aforementioned threshold to a constant level that is greater than the current Id that corresponds to the stable point A0 (indicated by a line L7 in the figure). In that case, when a short-circuit occurs immediately after the power MOSFET is turned on, as shown by a line L6, a certain amount of time is taken until the current reaches the threshold and the power loss at the power MOSFET is large and protection can not be attempted in that period.